The organs Bach knew and preferred had numerous 8' Principal stops. As the Principal tone is the single voice that is unique to the organ, and the 8' pitch the unison, the presence of 8' Principals seems logical.
Yet during the 'Organ Reform' movement, 8' Principals began to disappear from divisions other than the Great and Pedal. In many three manual organs designed by Ernest White for the Moller company, the only 8' Principal to be found at all would be a small scale 8' Spitz Principal for the Pedal. Imagine an organ of 40 ranks, with two Great mixtures and no 8' Principal tone!!
First Presbyterian Church - Anderson, South Carolina
M. P. Moller organ, 1958 - Opus 9225
8' Erzahler Celeste
1 1/3' Larigot
8' Gedeckt - extension
8' Gambe Celeste
III Plein Jeu
16' Sous Basse
16' Gedeckt - Swell
8' Gedeckt - Swell
4' Spitzprinzipal - extension
4' Rohrflote - extension
2' Rohrflote - extension
16' Posaune - extension Swell
The theory forwarded by Mr. White was this. In a Swell organ, an 8' Principal like tone could be created by combining a large scale 8' string with a small scale 8' stopped flute. This theory fails for a number of reasons:
It is a compromise at best. A large scale 8' string on the relatively low wind pressures of the era, looses its stringy character, and becomes neither string nor principal. The same is true for the stopped flute. If it is too small, the richness and fundamental are lost along with its inherent sweetness.
It creates an imbalance. Most organists prefer a Swell organ with 4' Principal, perhaps a 2' Principal and a mixture. A division such as this sounds top heavy and lacking proper foundation for the upper work. Many organists get around this problem by using the unison off and Swell to Swell 16' coupler, again a compromise.
1 + 1 = 3.
In a Swell organ of 8' String and 8' Stopped Flute, there are three possible combinations of 8' unison tone.
8' String alone
8' Stopped flute alone
8' String & 8' Flute
8' Principal alone
8' String alone
8' Flute alone
8' Principal & String
8' Principal & Flute
8' String & Flute
8' Principal, 8' String & 8' Flute
During the 1970's & 1980's, a secondary 8' Principal could be found in some organs, often in the Choir and called 'English Diapason'. While better than nothing, this also missed the point in my opinion. Unless the Choir organ were extensive, there would typically not be another 8' open stop or any volume, leaving the 8' English Diapason alone to function as string and principal without excelling at either.
The French have the Jeu de Fonds concept of 8' Montre, 8' Bourdon, 8' Flute Harmonique and in large instruments 8' Salicional. This concept should be looked to for inspiration in the design of any swell organ. Three stops, each of unique character, but which blend together to produce a wonderful, rich effect that greater than the sum of its parts.
There is a similar concept in French classic cooking called the mirepoix, a combination of three aeromatic vegetables, onion, celery and carrot. For the creole, it is called 'Le Trinitie' and is made up of; onion, celery and bell pepper instead of carrot. These three ingredients are the common building blocks upon which two entire cuisines are based. The musical combination of Principal, String, and Flute, while not sustaining for life as gumbo, can still be just as satisfying to the musical soul.